Seawater infiltration method and water infiltration intake unit

ABSTRACT

To clean sediments and the like trapped not only in a top layer of a sand filtration layer, but also in intermediate layers. A seawater infiltration method which uses a water infiltration intake unit which is formed in advance and provided with a water intake pipe embedded in a gravel layer which forms a deep layer of the sand filtration layer, and a backwashing pipe embedded in a sand layer which forms an intermediate layer and a surface layer of the sand filtration layer, and a water suction pipe which is disposed above the sand layer. A desired number of water infiltration intake units are combined to form a sand filtration layer at an installation site on an ocean floor, and they intake seawater from the sea which has undergone natural infiltration in the sand filtration layer and this is introduced into the water intake pipe. The seawater infiltration rate is set at less than 400 m/day. Water or air is injected from the backwashing pipe to agitate and blow upward from the surface layer living organisms or sediments trapped in intermediate layers of the sand filtration layer, and the agitated water is sucked in by a suction pipe and recovered. The seawater infiltration rate can be maintained as high as possible under 400 m/day.

TECHNICAL FIELD

The present invention relates to a filtration method employed for intake of seawater which infiltrates through a sand layer on an ocean floor, and a water infiltration intake unit for implementing the filtration method which has a backwashing pipe or the like which prevents clogging by removing living organisms or sediments which accumulate in a surface layer of the sand layer and which become trapped in intermediate layers.

BACKGROUND ART

As shown in FIG. 14, as an example of a present seawater intake method, a direct water intake method is used in which seawater is taken in from a water intake orifice 1 via a water conduit 2 provided on the ocean floor. In FIG. 14, Reference Numeral 3 is a pump for taking in the seawater, and Reference Numeral 4 is a reverse osmosis membrane system.

However, when employing the direct water intake method, debris, sediments, and living organisms are all taken in at the same time together with seawater, and thus there are cases in which water intake has to be stopped, for example, when there is abnormal adhesion of jellyfish or algal blooms, oil spill accidents, and increased turbidity due to high waves. Moreover, when employing the direct water intake method, it is necessary to perform periodic cleaning, to add chemicals (e.g., chlorine) to prevent adhesion, or to increase the diameter of pipes when living organisms becoming attached to the entire length of the pipes are taken into consideration, because the adhesion of sea life such as barnacles and mussels can be significant. In addition, when intake seawater is treated by reverse osmosis in the direct water intake method, a sand filtration system must be installed for filtering seawater to which a coagulant has been added, and thus there is a need to install a system for treating sludge which accumulates in the sand filtration system.

Accordingly, in recent years, attention has been focused on indirect water intake methods which take in seawater from a sand layer 5 (referred to below as a sand filtration layer) on the ocean floor, as shown in FIG. 15, without using chemicals such as coagulants to pre-treat the intake seawater.

As illustrated in FIG. 16, an indirect water intake method is a method which involves excavation of an ocean floor at an offshore site several hundred meters from a shoreline and at a depth of several tens of meters, forming a sand filtration layer 5 from supporting gravel layers 5 a and 5 b, and a filtration sand 5 c, and implementing backfilling up to the same ocean floor surface to install an intake pipe 6 in the supporting gravel layer 5 a, from which seawater which is purified by filtration is taken in. Although none of the problems of the direct water intake method arises when this indirect water intake method is employed, there are problems such as initial high cost and reduced water intake volume due to clogging at the infiltration surface, and consequently, this method has been slow in achieving widespread use.

A method is disclosed in Patent Reference 1 for achieving a stable water intake using a seawater infiltration intake method, which makes it possible to reduce as much as possible clogging of the sand filtration layer which takes in seawater on the ocean floor, and which makes it possible to remove sediments which accumulate on the surface of the sand filtration layer without a lot of labor.

The seawater infiltration intake method disclosed in Patent Reference 1 is characterized in that the seawater infiltration rate achieved in the sand filtration layer on the ocean floor is set at 1-8 m/day, and it is also characterized in that the water depth of the sand filtration layer is greater than the critical water depth for total sediment movement at which sand in the surface layer portion of the sand filtration layer travels at least 50 cm, and less than the critical water depth for surface layer movement at which the sand travels at least 1 cm.

However, in the seawater infiltration intake method disclosed in Patent Reference 1, a large surface area is needed for the intake of a large volume of seawater in a short period of time, because the seawater infiltration intake rate of 1-8 m/day is a very slow infiltration rate, and therefore requires a large-scale construction (Problem 1).

In addition, in the seawater infiltration intake method disclosed in Patent Reference 1, it is necessary to install the filtration intake system in the ocean area where the optimum flow of seawater is obtained, so as to prevent clogging of the sand filtration layer by silt (or sludge) which accumulates in the surface layer, thereby limiting it to sites where seawater is moved by waves (Problem 2).

Accordingly, in order to solve Problem 1 described above, the present applicant proposed a seawater infiltration method which increases the seawater infiltration intake rate to thereby greatly reduce the infiltration surface area and significantly reduce the scale of construction. However, the upper limit for the intake rate that could be realistically implemented was 400 m/day.

In addition, the present applicant proposed a seawater infiltration intake method which prevents clogging by manually removing living organisms or sediments which become trapped in the surface layer of the sand filtration layer. The present applicant also proposed a device for preventing clogging, which involved installing some type of a water-jet device such as a mechanical type or pneumatic type, for example, for removing the sediments trapped in the surface layer of the sand filtration layer. This made it possible to install a sand filtration layer in a calm ocean area where water is not moved rapidly by currents or waves, for example.

According to the seawater infiltration method proposed by the present applicant, the volume of intake water can be increased in a short period of time, and the water intake surface area can be reduced in comparison with the prior art, by setting the seawater infiltration rate as high as possible under 400 m/day. Furthermore, if a device for preventing clogging is installed in the surface layer of the sand filtration layer, there is no longer a need to install the infiltration intake system in the ocean area where the optimum seawater flow is obtained, thus making it possible to take in water near a seawater desalination plant. It is therefore possible to greatly reduce the scale of construction and the scale of the water intake system, and it is also possible greatly mitigate effects on the surrounding environment during construction.

However, in a method which utilizes the movement of seawater, or in the case of a method which injects water and the like from a clogging prevention device installed on the surface of the sand filtration layer, it is only possible to remove sediments which accumulate in the surface layer of the sand filtration layer, and it is not possible to remove living organisms and sediments trapped in intermediate layers which are deeper than the surface layer of the sand filtration layer. In particular, in cases where the seawater infiltration rate is set as high as possible under 400 m/day, clogging readily progresses in the intermediate layers of the sand filtration layer as well, so clogging occurs more frequently.

Accordingly, there is a possibility that the seawater infiltration rate will drop if only the sediments which accumulate in the surface layer of the sand filtration layer are removed.

In addition, in the seawater infiltration intake method disclosed in Patent Reference 1, the construction involved in installation is on a large scale, because the supporting gravel layer 5 a is formed in the excavated portion of the ocean floor and the intake pipe 6 is buried therein, and the supporting gravel layer 5 b and the filtration sand 5 c are formed on top of the supporting gravel layer 5 a, while implementing backfilling up to the same ocean floor surface, and all of this is done on site on the ocean floor. Moreover, if a defect occurs in a part of the intake pipe 6 after the system starts operating, the surface of the ocean floor will have to be excavated again to repair a malfunctioning part of the intake pipe 6. Patent Reference 1: Japanese Patent No. 3899788

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The problems which the present invention aims to solve are that the prior art seawater infiltration intake method relies on washing by the movement of seawater or by means of the clogging prevention device installed on the surface of the sand filtration layer and this is not able to remove living organisms or sediments trapped in the intermediate layers of the sand filtration layer, and there is a possibility of a reduced seawater infiltration rate. In addition, in the prior art method, installation involved large-scale construction. In the event that a defect occurs in a part of the intake pipe after the system starts operating, the surface of the ocean floor has to be excavated again to repair a malfunctioning part.

Means for Solving this Problem

The present invention has as its object to solve the above-described problems, and to provide a seawater infiltration method and a water infiltration intake unit which is able to remove not only living organisms and sediments which accumulate in the surface layer of a sand filtration layer, but also to remove living organisms and sediments which are trapped in intermediate layers, and the unit can be installed on an ocean floor with construction on a small scale, and it can be easily maintained.

The seawater infiltration method according to the present invention uses water infiltration intake units combined to form a sand filtration layer at an installation site on an ocean floor. Each of the water infiltration intake units comprises a water intake pipe embedded in a gravel layer which forms a deep layer of the sand filtration layer, and a backwashing pipe embedded in a sand layer which forms an intermediate layer and a surface layer of the sand filtration layer, and intakes seawater from the sea which has undergone natural infiltration in the sand filtration layer and been introduced into the water intake pipe. The method comprises

setting a seawater infiltration rate at less than 400 m/day, and

injecting water or air from the backwashing pipe to agitate and blow upward from the surface layer living organisms or sediments trapped in intermediate layers together with living organisms accumulated in the surface layer, thereby preventing clogging of the sand filtration layer.

According to the present method described above, living organisms or sediments trapped in intermediate layers of a sand filtration layer are agitated by air or water injected from a backwashing pipe, and these are blown above the sand filtration layer together with sediments present in the surface layer. The sediments which were blown upward in the sea above the sand filtration layer are dispersed to outside of the sand filtration layer by the movement of seawater produced by currents or waves.

The present invention described above makes it possible to easily form a sand filtration layer, by combining water infiltration intake units which were formed in advance and arranging them at an installation site on an ocean floor. In the event that a defect occurs in a part of the intake pipe after the system starts operating, the water infiltration intake unit which includes a malfunctioning part such as a pipe can be separated and replaced as a unit module, without needing to excavate the surface of the ocean floor to repair the malfunctioning part.

In the present invention described above, if the sand filtration layer is installed in a calm ocean area where water is not moved rapidly by currents or waves, clogging of the sand filtration layer may be prevented by forming water infiltration intake units in advance, being also equipped with a water suction pipe and installed on the upper portion of the sand filtration layer, and combining a desired number of these water infiltration intake units at the installation site on the ocean floor, injecting water or air from the backwashing pipe to agitate and blow upward from the surface layer living organisms and sediments trapped in intermediate layers together with living organisms and sediments accumulated in the surface layer, after which the agitated water containing the living organisms and sediments is sucked in by the water suction pipe.

Advantageous Effects of the Invention

According to the present invention, clogging can be prevented by removing living organisms or sediments accumulated in the surface layer and trapped in the intermediate layers of the sand filtration layer, and continuous high-speed filtration can be achieved, by maintaining the seawater infiltration rate as high as possible under 400 m/day. Moreover, when water infiltration intake units of the present invention are combined to form the sand filtration layer, the scale of construction during installation in greatly reduced. In addition, it becomes easy to maintain the system, because after the system starts operating, a water infiltration intake unit with a problem can be separated and replaced as a unit module.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating an example of a water infiltration intake unit used in the seawater infiltration method of the present invention, in a case where the unit is of a size that can be mounted on a truck. FIG. 1( a) is a sectional view along the line A-A′ of a planar view; FIG. 1( b) is a sectional view along the line B-B′ of a planar view; and FIG. 1( c) is a drawing viewed from a planar direction.

FIG. 2 is a drawing illustrating the various pipes of the water infiltration intake unit of the present invention viewed from a planar direction. FIG. 2( a) is a planar view of the water suction pipe; FIG. 2( b) is a planar view of the backwashing pipes; and FIG. 2( c) is a planar view of the water intake pipe.

FIG. 3 is a drawing illustrating examples of arrangement of water intake pipes of the water infiltration intake units of the present invention. FIG. 3( a) illustrates an arrangement in a case where water intake pipes in 5 blocks are connected in a bus-type arrangement, and FIG. 3( b) illustrates an arrangement in a case where intake pipes in 5 blocks are connected to respective pump pits.

FIG. 4 is a drawing illustrating an example of the connection of backwashing pipes of the water infiltration intake units of the present invention. FIG. 4( a) illustrates the connection of a pump when a construction is used in which water is injected, and FIG. 4( b) illustrates the connection to an air compressor when a construction is used in which air is injected.

FIG. 5 is a drawing illustrating examples of the dimensions and arrangement of the water infiltration intake units of the present invention. FIG. 5( a) illustrates an example when the water intake volume is set at 100,000 t/day, and FIGS. 5( b) and (c) illustrate an example when the water intake volume is set at 400,000 t/day, and another example.

FIG. 6 is a drawing illustrating an example of a water infiltration intake unit of the present invention which uses a water discharge pipe. FIG. 6( a) is a sectional view along line A-A′ of a planar view; FIG. 6( b) is a sectional view along the line B-B′ of a planar view; and FIG. 6( c) is a drawing viewed from a planar direction.

FIG. 7 is a drawing illustrating another example of a water infiltration intake unit of the present invention which uses a water discharge pipe. FIG. 7( a) is a drawing of a backwashing pipe viewed from a planar direction, and FIG. 7( b) is a schematic drawing of a branch pipe of a water discharge pipe viewed from a cross-sectional direction, and the drawing illustrates the angle of injection of water from a water discharge pipe.

FIG. 8 is a drawing illustrating an example of the arrangement of water intake pipes and backwashing pipes in the direction of height within a unit module of a water infiltration intake unit of the present invention.

FIG. 9 is a drawing illustrating an example of a water infiltration intake unit of the present invention which is not provided with water suction pipes or water discharge pipes. FIG. 9( a) is a sectional view along the line A-A′ of a planar drawing; FIG. 9( b) is a sectional view along the line B-B′ of a planar drawing; and FIG. 9( c) is a drawing viewed from a planar direction.

FIG. 10 is a drawing illustrating an experimental flow of a seawater infiltration method.

FIG. 11 is a graph showing experimental results. FIG. 11( a) is a graph showing turbidity data, and FIG. 11( b) is a graph showing silt density index (SDI) data.

FIG. 12 is a graph showing the relationship between the passage of time and the loss of pressure in the case of total blockage and in the case of standard blockage.

FIG. 13 is a drawing illustrating the seawater infiltration method of the present invention.

FIG. 14 is a schematic drawing illustrating a direct seawater intake method of the prior art.

FIG. 15 is a schematic drawing illustrating an indirect water intake method of the prior art.

FIG. 16 is a schematic structural diagram of ocean floor filtration elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention, the object of preventing clogging of the sand filtration layer to maintain the seawater infiltration rate as high as possible under 400 m/day is achieved by agitating the living organisms or sediments trapped in the intermediate layers of the sand filtration layer and blowing them upward from the surface layer by means of water or air injected from a backwashing pipe, together with the living organisms or sediments accumulated in the surface layer, in order to continually implement high-speed filtration.

It desirable that clog-causing substances which are contained in the living organisms and sediments which are blown above the sand filtration layer are recovered by a water suction pipe provided at the top of the sand layer, so that these substances do not have a negative effect on the environment surrounding the sand filtration layer.

In the case of ocean areas where there is relatively little need to protect the environment, there are instances in which it is permitted to discharge into the surrounding area of the sand filtration layer, clog-causing substances including the living organisms and sediments which are blown above the sand filtration layer. In cases where a sand filtration layer is installed in such an ocean area, one of the configurations described below should be selected, depending on the flow rate of the seawater.

If installation is done in an ocean area where the flow of seawater is slow, a structure is used whereby clog-causing substances are manually discharged by means of a discharge pipe provided above the sand layer. On the other hand, if installation is done in an ocean area where the flow of seawater is rapid, clog-causing substances are dispersed by the movement of seawater produced by currents or waves, so a structure may be employed which is not provided with water suction pipes or water discharge pipes.

EXAMPLES

An embodiment of the present invention is described in detail below, using FIG. 1 to FIG. 13.

FIG. 1 is a drawing illustrating an example of a water infiltration intake unit 11 used in the seawater infiltration method of the present invention.

In FIG. 1( a) and FIG. 1( b), Reference Numeral 12 is a water intake pipe embedded in a gravel layer 13 which forms a deep layer of a sand filtration layer on the ocean floor. The water intake pipe 12 is formed from a main pipe 12 a and a plurality of branch pipes 12 b which branch in a direction crossing the main pipe 12 a. Embedded in a sand layer 15 formed from intermediate layers 15 b and 15 c and a surface layer 15 a of a sand filtration layer is a backwashing pipe 14 formed from a main pipe 14 a and a plurality of branch pipes 14 b which branch in a direction crossing the main pipe 14 a.

In FIG. 1( c), Reference Numeral 16 is a water suction pipe formed from a main pipe 16 a and a plurality of branch pipes 16 b. As shown in FIG. 1( a), the main pipe 16 a is constructed between mutually facing side surfaces 17 a and 17 c of a frame 17. As shown in FIG. 1( b), the end portions of the plurality of branch pipes 16 b which branch in a direction crossing the main pipe 16 a are supported by side surfaces 17 b and 17 d of the frame 17. According to such a configuration, the water suction pipe 16 is installed above the sand layer 15, being spaced by a distance from the upper surface of the sand layer 15.

The water infiltration intake unit 11 of the present example is constructed with the frame 17 which measures, for example, 10 m in length×2.5 m in width×2.5 m in height, and which houses the water intake pipe 12 embedded in a central position in the direction of height of the gravel layer 13 which has a height of 0.5 m, the backwashing pipe 14 embedded in an upper portion of in the direction of height of the sand layer 15 which has a height of 2.0 m, and the water suction pipe 16 disposed above the upper surface of the sand layer 15. Accordingly, the water infiltration intake unit 11 can be transported on a truck having a platform on which a load of the above-described size can be mounted, making it easy to transport on land. An optimal material for the frame 17 may be selected from FRP, concrete, metal, or the like, depending on the water quality in the ocean area where the sand filtration layer is installed, and depending on the components of the substances contained in the seawater.

FIG. 2 is a drawing illustrating the various pipes of the water infiltration intake unit 11, as viewed from a planar direction. The lower portion of the drawing is a land side where a seawater desalination plant is provided, and the upper portion of the drawing is an ocean side.

As shown in FIG. 2( a), there are provided in the branch pipes 16 b of the water suction pipe 16 a plurality of suction holes 16 ba and 16 bb arranged in rows on the land side and on the ocean side. In the present example, the suction holes 16 ba on the land side and the suction holes 16 bb on the ocean side open parallel to a lengthwise direction of the main pipe 16 a, and when installed on site, they are oriented so that the blowing angle of the blow holes are oriented parallel to the horizontal direction. The land side of the main pipe 16 a is connected to a water collection pump of the seawater desalination plant. Turbid water sucked in by the suction holes 16 ba and 16 bb is collected in the main pipe 16 a from the branch pipes 16 b, and recycled to the seawater desalination plant.

As shown in FIG. 2( b), a plurality of blow holes 14 ba are arranged in rows in the branch pipes 14 b of the backwashing pipe 14 in a position on the side facing upward when installed on site. In the present example, the blowing angle of the blow holes 14 ba is 90° with respect to the horizontal position. The land side of the main pipe 14 a is connected to a water supply pump or an air compressor of the seawater desalination plant. Water or air fed from the main pipe 14 a to the branch pipes 14 b is blown from the blow holes 14 ba upwardly in a perpendicular direction. In the present example, the blow holes 14 ba are provided in a direction facing upward when installed on site, for instance, but the blow holes 14 ba of the backwashing pipe 14 may be configured so as to face downward when installed on site.

The branch pipes 12 b of the water intake pipe 12 are provided with a plurality of water intake holes on the entire surface area (not shown in FIG. 2( c)). The land side of the main pipe 12 a is connected to a water collection pump of the seawater desalination plant. Seawater which has undergone natural infiltration in the sand filtration layer is introduced into the branch pipes 12 b by the water intake holes, and taken into the seawater desalination plant via the main pipe 12 a.

The seawater infiltration method according to the present invention uses the above-described water infiltration intake units 11 which are built on land in advance, and a certain number of water infiltration intake units 11 are combined to form a sand filtration layer at an installation site on an ocean floor. Seawater which has undergone natural infiltration in the sand filtration layer in the ocean is introduced into the water intake pipe 12, and the seawater infiltration rate is set at less than 400 m/day. Living organisms or sediments trapped in the intermediate layer 15 b, which is above the backwashing pipe 14, are agitated by blowing water or air from the blow holes 14 ba of the backwashing pipe 14 upward at an angle of +90° or downward at an angle of −90° with respect to a horizontal plane, thereby blowing them upward in the ocean above the sand filtration layer, together with living organisms accumulated in the surface layer of the sand filtration layer. After that, the agitated water containing the living organisms or sediments is sucked in by the water suction pipe 16 installed above the surface layer of the sand filtration layer. By operating in this manner, the living organisms or sediments which are blown upward above the sand filtration layer are recovered without accumulating again on the surface of the sand filtration layer, thereby making it possible to reliably prevent clogging of the sand filtration layer.

The living organisms or sediments which are blown upward above the sand filtration layer not only contain clog-causing components such as silt, but also contain substances with a particle diameter suitable for supporting the seawater filtration effect in the sand filtration layer.

Accordingly, in this example, using a difference in settling rates of the living organisms or sediments blown upward by water or air blown from the backwashing pipe 14 into the seawater above the sand filtration layer, the agitated water is sucked in by the water suction pipe 16 according to a timing at which the clog-causing substances settle in the sand filtration layer. It is therefore possible in this example to leave in the sand filtration layer substances which serve to support the filtration effect by sucking out only those clog-causing substances.

Following is a specific example of a method of installing and connecting the water infiltration intake units 11 of the present invention, with regard to the size of a water intake area formed by assembling a water infiltration intake block 110.

FIG. 3 is a drawing illustrating examples of arrangement of water intake pipes 12 of the water infiltration intake units 11. In this example, a water intake pipe block 120 is formed by arranging horizontally 7 water intake pipes 12. In the case of such a configuration, the water intake pipe blocks 120 may be connected to a water collection pump which integrates 5 of the water intake pipe blocks 120 into a single unit by using a shared pipe 18, as shown in FIG. 3( a). Alternatively, each of the water intake pipe blocks 120 may be separately connected to a water collection pump set 19.

FIG. 4 is a drawing illustrating an example of the connection of backwashing pipes 14 of the water infiltration intake units 11. In this example, a backwashing pipe block 140 is formed by arranging 7 backwashing pipes 14 in parallel in a row. In the present invention, water or air may be injected from the backwashing pipes 14, in order to agitate living organisms or sediments accumulated in the surface layer of the sand filtration layer and trapped in intermediate layers and to blow them into the sea above the sand filtration layer. If a structure is used for injecting water, the backwashing pipe block 140 is connected to a water supply pump 20, as shown in FIG. 4( a). On the other hand, if a structure is used for injecting air, then the backwashing pipe block 140 is connected to an air compressor 21.

FIG. 5 is a drawing illustrating examples of the dimensions and arrangement of the water infiltration intake units 11 of the present invention. In the example shown in FIG. 5( a), the structure consists of water infiltration intake blocks 110 constructed with 8 or 9 water infiltration intake units 11 which are each 10 m in length in the lengthwise direction of the main pipes 12 a, 14 a, and 16 a and are arranged in parallel in a row. Then, four of these water infiltration intake blocks 110 are arranged in parallel in a row to form a water intake area 1100 having a size 10 m×105 m (not including the sheet thickness of the frame).

In the present invention, the seawater infiltration rate is set at less than 400 m/day, and if the seawater infiltration rate is set at 100 m/day, and if the infiltration surface area is set at 30 m² per each unit module of the water infiltration intake units 11, then the amount of water collected would be 3,000 m³/day per each unit module of the water infiltration intake units 11. In a case where the water intake area 1100 is formed from about 35 water infiltration intake units 11, as in the example shown in FIG. 5( a), the total amount of water collected would be about 100,000 t/day. This is about the scale of water intake at the “Mamizu Pia” seawater desalination facility in Fukuoka Prefecture, Japan.

If the daily water intake volume needs to be increased to 400,000 t/day, for example, then four water intake areas 1100 of FIG. 5( a) may be arranged as illustrated in FIG. 5( b) and FIG. 5( c). In this case, it is possible to obtain a water intake volume of 400,000 t/day using a water intake area of 25 m×ca 270 m in the example of FIG. 5( b), and 210 m×ca 25 m in the example of FIG. 5( c), and these are much smaller than in the conventional osmotic water intake method.

In the present invention, the water intake pipe 12, the gravel layer 13, the backwashing pipe 14, the sand layer 15, and the water intake pipe 15 are formed into a unit, and an optimal arrangement can be selected according to the topology of the ocean area where installation takes place. It is also possible to accommodate a required daily water intake volume by varying the number of units combined as described above.

According to the present invention, if damage occurs in some of the water infiltration intake units 11, the system as a whole affected little, because it is necessary to replace only the damaged water infiltration intake units 11. Moreover, the maintenance construction is on a small scale, and the maintenance can be reduced, because there is no need to excavate the surface of the ocean floor when replacing the water infiltration intake units 11.

FIG. 6 is a drawing illustrating an example of a water infiltration intake unit of the present invention in a case where a water discharge pipe is used as a means to remove living organisms or sediments which are blown upwards in the ocean. In the description given below, only the items which are different from the construction of example of FIG. 1 which uses the water suction pipe 16 are explained.

In FIG. 6, Reference Numeral 22 is a water discharge pipe formed from a main pipe 22 a and a plurality of branch pipes 22 b. As shown in FIG. 6( a), the main pipe 22 a is constructed crosswise between the opposite side surfaces 17 a and 17 c of the frame 17. Moreover, as shown in FIG. 6( b), the end portions of the plurality of branch pipes 22 b which branch in a direction crossing the main pipe 22 a are supported by side surfaces 17 b and 17 d of the frame 17. According to such a configuration, the water discharge pipe 22 is installed above the sand layer 15, being spaced apart by a distance from the upper surface of the sand layer 15.

The branch pipes 22 b are provided with a plurality of injection holes on the ocean side (these are not depicted in FIG. 2( b) because the drawing is viewed from the land side). The injection holes open in parallel in a lengthwise direction of the main pipe 22 a, and when installed on site, they are oriented so that the suction angle of the suction holes are oriented parallel to the horizontal direction. The land side of the main pipe 22 a is connected to a water supply pump of the seawater desalination plant. Water fed to the branch pipes 22 b by the main pipe 22 a is injected from the injection holes in a horizontal direction to the ocean side.

Accordingly, in this example, living organisms or sediments trapped in the intermediate layers of the sand filtration layer are agitated by water or air injected from the backwashing pipe 14 and blown upward above the surface layer, together with living organisms accumulated in the surface layer, after which they are dispersed to outside of the sand filtration layer by water injected from the water discharge pipe 22.

FIG. 7 is a drawing illustrating another example of a water infiltration intake unit of the present invention which uses a water discharge pipe, wherein the water discharge pipe 23 is viewed from a planar direction. The right side of the drawing is the land side where a seawater desalination plant is installed, and the left side of the drawing is the ocean side (in the direction of the arrows). In this example, there are no other water infiltration intake units adjacently disposed on the ocean side.

As shown in FIG. 7( a), a plurality of injection holes 23 bb are arranged in rows only on the ocean side in the branch pipes 23 b of the water exhaust pipe 23. FIG. 7( b) is a schematic drawing of a branch pipe 23 b of a water discharge pipe 23 viewed from a cross-sectional direction, and the drawing illustrates the angle of injection θ of water from an injection hole 23 bb. In this example, the angle of injection θ can be variably set within a range of 30-60° with respect to a horizontal plane by installing a nozzle at the injection hole 23 bb. The land side of the main pipe 23 a is connected to a water supply pump of a seawater desalination plant. For example, if the angle of injection of the injection hole 23 bb is set at 45°, then water fed from the main pipe 23 a to the branch pipe 23 b is injected upward from the injection hole 23 bb on the ocean side at a 45° angle with respect to a horizontal plane.

Accordingly, in this example, the water discharge pipe 23 is formed so as to inject water in a direction other than the direction in which the water infiltration intake units are adjacent, and the angle of injection of the water is in a range of 30-60° with respect to a horizontal plane.

Therefore, in this example, clog-causing sediments and the like which are discharged by the discharge pipe 23 do not settle on top of the sand filtration layer of other water infiltration intake units. Moreover, in the present example, water injected from the injection hole 23 bb forms a parabolic curve, thereby making it possible to discharge to a greater distance the clog-causing sediments and the like.

Incidentally, if the living organisms or sediments which accumulate on the ocean floor are unnecessarily discharged into the surrounding area, they can affect the surrounding natural environment in some manner.

Accordingly, in this example, using differences in settling rates of the living organisms or sediments blown upward by water or air blown from the backwashing pipe 14 into the seawater above the sand filtration layer, water is injected from the water discharge pipe 23 according to a timing at which the clog-causing substances settle in the sand filtration layer. Thus, in the present example, it is possible to have very little effect on the surrounding natural environment.

FIG. 8 is a drawing illustrating an example of the arrangement of the water intake pipes 12 and the backwashing pipes 14 in the direction of height within a unit module of a water infiltration intake unit 11 of the present invention. In the present invention, the water infiltration intake unit 11 should not be too far from a bottom surface 17 e of the frame 17 of the water infiltration intake unit 11, so as not to reduce the filtration capacity. Specifically, if the outer diameter of the water infiltration intake unit 11 is set at D, the COP (center of pipe) height of the water intake pipe 12 should be in a range of 0.75 D to 1.25 D above the bottom surface 17 e.

The backwashing pipe 14 is advantageously installed at a position as deep as possible, so as to achieve a broad range of backwashing of the sediments accumulated in the surface layer or trapped in the intermediate layers of the sand filtration layer. However, if the installation position is too deep, a high water pressure is required, so consideration must be given to balancing these two. Specifically, if the outer diameter of the backwashing pipe 14 is set at d, the COP (center of pipe) height of the backwashing pipe 14 is in a range of 1.0 d to 5.0 d below a surface 15 d of the sand layer of the sand filtration layer.

FIG. 9 is a drawing illustrating an example of a water infiltration intake unit of the present invention which uses the movement of seawater as a means for removing living organisms or sediments blown upward in the seawater. Except for the fact that there is no water suction pipe 16 or no water discharge pipe 22, the structure of FIG. 9 is identical to that of the example of FIG. 1. In this example, the living organisms and sediments trapped in the intermediate layers of the sand filtration layer are blown upward from the surface layer by water or air injected from the backwashing pipe 14, together with the living organisms and sediments accumulated on the surface of the sand filtration layer, and dispersed to outside of the sand filtration layer by the movement of seawater produced by currents or waves.

Following is an explanation of the reason why the seawater infiltration rate is set at under 400 m/day in the seawater filtration method of the present invention.

FIG. 10 is a diagram illustrating an experimental flow of a seawater infiltration method of the present invention. In FIG. 10, Reference Numeral 31 is a water intake pump immersed at a position 50 cm from the ocean floor and 3.3 m from the surface of the water. Reference Numeral 32 is a raw water tank which holds seawater which is drawn up by the water intake pump 31. The seawater held in the raw water tank 32 is drawn up by a raw water pump 33, and fed to a column device 34. The column device 34 is provided with a filtration layer formed from a sand layer 34 a and a gravel layer 34 b, and filtered water which has passed through this filtration layer is guided to a treated water tank 35.

In the experimental flow shown in FIG. 10, there are provided a reverse conduit pipe 37 which sends back the filtered water from the treated water tank 35 to the column device 34 using an interposed reverse-direction pump 36. An overflow pipe 38 is also provided to guide the seawater fed to the column device 34 to the treated water tank 35 so it does not overflow.

The turbidity and silt density index SDI of the filtered water are measured after the seawater is taken in by the water intake pump of the experimental flow device and filtered through a filtration layer of the column device. The filtration layer used in this measurement consists of a 0.45 mm diameter sand layer (thickness 900 mm), a 2-4 mm diameter gravel layer (thickness 75 mm), a 4-8 mm gravel (thickness 75 mm), and a 6-12 mm gravel (thickness 150 mm).

The results of measurement are given in FIG. 11. To the raw water for which turbidity data is obtained is added in advance a quantity of silt such that the turbidity is that for a water infiltration intake rate of 0 m/day shown in FIG. 11( a). If the seawater infiltration rate is set at 50-400 m/day, the turbidity and silt density index SDI do not change from the conventional case where the seawater infiltration rate is set at 1-8 m/day, which confirms that the same treatment capacity was indicated.

Incidentally, in the invention disclosed in Patent Reference 1, the seawater infiltration rate occurring in the sand filtration layer on the ocean floor is set at 1-8 m/day, and the sand filtration layer is set at a depth greater than the critical water depth for total sediment movement at which sand in the surface layer portion of the sand filtration layer travels at least 50 cm, and less than the critical water depth for surface layer movement at which the sand travels at least 1 cm.

Following is an explanation of the reason why the conditions under which the seawater infiltration rate occurs are such that the sand filtration layer is set at a depth greater than the critical water depth for total sediment movement at which sand in the surface layer portion of the sand filtration layer travels at least 50 cm, and less than the critical water depth for surface layer movement at which the sand travels at least 1 cm in the invention disclosed in Patent Reference 1.

The reason why the sand in the surface of the sand filtration layer travels at least 1 cm at the critical water depth for total sediment movement, which is the maximum water depth at which some degree of sand particle movement resulting from waves can be observed on the surface of the ocean floor, is that it is the level at which sand on the ocean floor can be washed, and if it is at a greater water depth, then there is almost no movement of sand particles of the surface of the sand filtration layer.

On the other hand, the reason why sand in the surface of the sand filtration layer travels at least 50 cm at the critical water depth for total sediment movement, which is the maximum water depth at which the sand filtration layer on the ocean floor can be observed to be eroded by wave action, is because erosion of the sand filtration layer on the ocean floor is observed.

Moreover, in Patent Reference 1, the diameter of silt particles is generally about 0.005-0.074 mm, and the flow rate of seawater in which silt does not start moving is determined, that is, the critical flow rate for movement is determined. The critical flow rate for movement is obtained by multiplying the surface area porosity (=0.35) by the actual critical flow rate of the silt particles. According to a graph showing the relationship between the particle diameter and the actual critical flow rate, the actual critical flow rate was found to be 0.026 cm/s when the particle size of silt was 0.08 mm.

Therefore, the upper limit of the critical flow rate for movement of silt is 0.026×0.35×24×3600=786.24 cm/day. Based on these results, the maximum seawater infiltration rate should be 8 m/day, in order to prevent clogging due to silt blown upward within the sand filtration layer.

Further, the seawater infiltration rate must be at least 1 m/day, so as to supply sufficient oxygen to the sand filtration layer to prevent the annihilation of biofilms.

Accordingly, in the invention according to Patent Reference 1, setting the seawater infiltration rate at 1-8 m/day under the above conditions makes it possible to remove refuse and sediments such as silt accumulated in the sand filtration layer by suitable agitating the surface layer of the sand filtration layer with waves and currents, thereby making it possible to secure a stable intake of water.

The upper limit of the seawater infiltration rate specified in the invention according to Patent Reference 1 is a condition which was set in order to prevent penetration or admixture of silt in the sand filtration layer on the top layer of the ocean floor. Following up on the 8 m/day limit on silt absorption in Patent Reference 1, the present inventors confirmed that comparable treatment performance was exhibited as when the water infiltration intake rate is 1-8 m/day, for example when the water infiltration intake rate is 400 m/day, and there is a tendency for silt to be absorbed.

In other words, it can be conjectured that if silt is not agitated or if there is no flow field, and if the water infiltration intake rate is set at 400 m/day, then the absorption rate of silt to the sand filtration layer is (water infiltration intake rate cm/s)×(porosity resulting from the sand particle diameter). Therefore, if the water infiltration intake rate is 400 m/day, then the absorption rate of silt into the sand filtration layer is {40,000 cm/(24×3,600)}×0.35=0.16 cm/s. In order to achieve a critical flow rate of at least 1.0 m/day to supply oxygen, the absorption rate of silt into the sand filtration layer should be {100 cm/(24×3,600)}×0.35=0.0004 cm/s.

In the above computation, if the seawater infiltration intake rate is set at 400 m/day, it becomes difficult to achieve washing which is feasible, because silt particles which cause clogging enter the sand filtration layer at a rate of about 6 m/hr (about 0.16×3,600/100).

However, when the silt moves together with the water, there results what is known as a standard blockage type, because the silt particles are much smaller than the voids in the filtration sand, so the silt adheres in the vicinity of the surface layer and remains, since it is detained by the intermolecular forces (physical adhesion, static electricity) of the filtration sand and accumulates.

In the previously described experiment in which silt components are added, with the results given in FIG. 11( a), under conditions where the water infiltration intake rate is set at under 400 m/day, after 2 hours, it was observed that most of the silt accumulated in the surface layer, and there was penetration on the order of only 1 cm inside the sand filtration layer.

Unlike total blockage which occurs when the silt particles are larger than the voids in the filtration sand, standard blockage occurs such that it takes time for the voids to become smaller due to adhesion of silt particles, since the particles completely block the voids, as shown in FIG. 12. This means that even if silt continues to be removed, infiltration is possible for a long period of time, because a gradual loss in pressure occurs up to the void retention threshold value. The amount of time which passes depends on the condition of the filtration material and the condition of the seawater (silt density), and is an important factor in determining the intervals of forced washing.

Based on the experimental results and findings by the inventors as described above, the present invention achieves a significant reduction in the scale of construction and the scale of water intake facilities by increasing the rate of seawater infiltration through the filtration material to a rate which had heretofore been commonly considered taboo.

According to the results of experiments performed by the inventors involving subterranean water which has greater infiltration characteristics than seawater, normal continuous operation was possible up to an infiltration rate of 600 m/day. However, if the infiltration rate was set at 700 m/day, the amount of water needed for cleaning exceeded the water intake volume, so water could not continuously flow in the sand filtration layer, and normal water intake was impossible. Therefore, in the present invention, which employs a filtration method when there is seawater infiltration intake, the safety factor is set at about 1.5, and the upper limit for the seawater infiltration rate is set at 400 m/day.

For the above reason, according to the seawater infiltration method of the present invention, the seawater infiltration rate is set at less than 400 m/day, during seawater infiltration intake when seawater which has undergone natural infiltration in a sand filtration layer is introduced into a water intake pipe, the seawater infiltration rate is set at less than 400 m/day.

In the present invention, when the seawater infiltration rate is set at 400 m/day, the water intake volume is 50 times that of the prior art in which the seawater infiltration rate was 8 m/day, thereby making it possible to achieve a water intake surface area 1/50 that of the prior art. In addition, installation is no longer necessary in ocean areas in which an optimal flow of seawater is accelerated. As shown in FIG. 13, a water infiltration intake facility can be installed in the vicinity of a water desalination plant 41, thereby making it possible to significantly reduce the scale of construction and the scale of water intake facilities, and also making it possible to greatly mitigate effects on the surrounding environment during construction.

Moreover, according to the seawater infiltration method of the present invention, the seawater infiltration rate can be maintained as high as possible under 400 m/day, and a high-speed filtration can be implemented continuously, because clogging can be more reliably prevented by removing living organisms and sediments not only from the surface layer of the sand filtration layer, but also by removing living organisms and sediments which are trapped in the intermediate layers.

The present invention is not limited to the above-described examples, and the preferred embodiment may, of course, be advantageously modified within the scope of the technical ideas recited in the claims.

For example, in the example described above, there was disclosed a case in which the water infiltration intake units 11 were formed in advance, but in the seawater infiltration method of the present invention, it is also possible to form a sand filtration layer with the water intake pipe 12 and the backwashing pipe 14 embedded at the installation site on the ocean floor, without using the water infiltration intake units 11.

In detail, the seawater infiltration method comprises embedding a water intake pipe in a deep layer of a sand filtration layer, embedding a backwashing pipe, which injects water or air, in the intermediate layers of the sand filtration layer, and intaking seawater from the sea which has undergone natural infiltration in the sand filtration layer and been introduced into the water intake pipe. The method further comprises

setting a seawater infiltration rate at less than 400 m/day, and

injecting water or air from the backwashing pipe to agitate and blow upward from the surface layer living organisms or sediments trapped in intermediate layers together with living organisms accumulated in the surface layer, thereby preventing clogging of the sand filtration layer.

In cases in which water infiltration intake units are not used, as described above, a water suction pipe is further installed above the surface layer of the sand filtration layer, and water or air are injected from the backwashing pipe to agitate and blow upward from the surface the living organisms or sediments trapped in the intermediate layers of the sand filtration layer together with living organisms or sediments accumulated on the surface of the sand filtration layer. After that, the agitated water containing the living organisms or sediments is sucked in by the water suction pipe, thereby making it possible to prevent clogging of the sand filtration layer without having a negative effect on the surrounding environment.

Alternatively, another water discharge pipe is installed above the surface layer of the sand filtration layer, and water or air are injected from the backwashing pipe to agitate and blow upward from the surface the living organisms or sediments trapped in the intermediate layers of the sand filtration layer together with living organisms or sediments accumulated on the surface of the sand filtration layer. After that, water is injected from the water discharge pipe, thereby making it possible to prevent clogging of the sand filtration layer, even in calm ocean areas where there is little movement of the seawater.

In the above example there was disclosed a structure whereby the intermediate layers of the sand filtration layer were cleaned by injecting water or air from the backwashing pipe 14. However, a structure is also possible whereby the intermediate layers (sand layers) are cleaned by the backwashing pipe 14, and the deep layers (gravel layers) of the sand filtration layer are cleaned by reversing the flow of water at a desired timing with respect to the water intake pipe 12 and injecting water from the intake holes of the branch pipe 12 b.

In the above example, an example using the water suction pipe 16 and an example using the water discharge pipes 22 and 23 were separately described. However, a structure is also possible in which the function of the water suction pipe and the function of the water discharge pipe are provided together in a single device, by switching a water collection and a water feed of a pump installed in a seawater desalination plant.

The filtration material used in the gravel layers and the sand layers of the water infiltration intake units of the present invention is not limited to natural gravel and sand, regardless of quality. For example, artificial particulate ceramics or artificial glass, which have little effect on the environment, may be used as filter materials in the gravel layers or the sand layers. If such artificial filtration materials are used, there is generally a problem of greater cost, but the seawater infiltration method of the present invention differs from the prior art in that the water intake surface area can be significantly reduced, thus making it easier to use artificial filtration materials such as those described above.

EXPLANATION OF THE REFERENCE SYMBOLS

11 Water infiltration intake unit

12 Water intake pipe

13 Gravel layer

14 Backwashing pipe

15 Sand layer

16 Water suction pipe

22 Water discharge pipe

23 Water discharge pipe 

1. A seawater infiltration method which uses water infiltration intake units combined to form a sand filtration layer at an installation site on an ocean floor, wherein each of the water infiltration intake units comprises a water intake pipe embedded in a gravel layer which forms a deep layer of the sand filtration layer, and a backwashing pipe embedded in a sand layer which forms an intermediate layer and a surface layer of the sand filtration layer, and intakes seawater from the sea which has undergone natural infiltration in the sand filtration layer and been introduced into the water intake pipe, comprising: setting a seawater infiltration rate at less than 400 m/day, and injecting water or air from the backwashing pipe to agitate and blow upward from the surface layer living organisms or sediments trapped in intermediate layers together with living organisms accumulated in the surface layer, thereby preventing clogging of the sand filtration layer.
 2. The seawater infiltration method according to claim 1, wherein the water infiltration intake units are further provided with suction pipes disposed above the sand layer, and are combined to form a sand filtration layer at an installation site on an ocean floor, the method further comprising: injecting water or air from the backwashing pipe to agitate and blow upward from the surface layer living organisms or sediments trapped in intermediate layers together with living organisms accumulated in the surface layer, after which the agitated water containing the living organisms or sediments is sucked in by the water suction pipes, thereby preventing clogging of the sand filtration layer.
 3. The seawater infiltration method according to claim 2, further comprising using a difference in settling rates of the living organisms or sediments blown above the sand filtration layer when sucking in the agitated water by the water suction pipes according to a timing at which the substances which are to be sucked in settle in the sand filtration layer.
 4. The seawater infiltration method according to claim 1, wherein the water infiltration intake units are further provided with water discharge pipes disposed above the sand layer, and are combined to form a sand filtration layer at an installation site on an ocean floor, the method further comprising: injecting water or air from the backwashing pipe to agitate and blow upward from the surface layer living organisms or sediments trapped in intermediate layers together with living organisms accumulated in the surface layer, after which the agitated water containing the living organisms or sediments is injected by the water discharge pipes to discharge it to outside of the sand filtration layer, thereby preventing clogging of the sand filtration layer.
 5. The seawater infiltration method according to claim 4, further comprising using a difference in settling rates of the living organisms or sediments blown above the sand filtration layer to inject water from the water discharge pipes according to a timing at which the substances which are to be discharged to outside of the sand filtration layer.
 6. A water infiltration intake unit used in the seawater infiltration method according to claim 4, wherein the water discharge pipe injects water in a direction other that in which the water infiltration intake units are adjacent, and the angle of injection of the water is in a range of 30-60° with respect to a horizontal plane.
 7. A seawater infiltration method which uses a water intake pipe embedded in a deep layer of a sand filtration layer and a backwashing pipe which injects water or air and which is embedded in the intermediate layers of the sand filtration layer, and intakes seawater from the sea which has undergone natural infiltration in the sand filtration layer and been introduced into the water intake pipe, the method comprising: setting a seawater infiltration rate at less than 400 m/day, and injecting water or air from the backwashing pipe to agitate and blow upward from the surface layer living organisms or sediments trapped in intermediate layers together with living organisms accumulated in the surface layer, thereby preventing clogging of the sand filtration layer.
 8. The seawater infiltration method according to claim 7, wherein a water suction pipe is further installed above the surface layer of the sand filtration layer, and water or air are injected from the backwashing pipe to agitate and blow upward from the surface the living organisms or sediments trapped in the intermediate layers of the sand filtration layer together with living organisms or sediments accumulated on the surface of the sand filtration layer, after which the agitated water containing the living organisms or sediments is sucked in by the water suction pipe, thereby preventing clogging of the sand filtration layer.
 9. The seawater infiltration method according to claim 7, wherein a water discharge pipe is further installed above the surface layer of the sand filtration layer, and water or air are injected from the backwashing pipe to agitate and blow upward from the surface the living organisms or sediments trapped in the intermediate layers of the sand filtration layer together with living organisms or sediments accumulated on the surface of the sand filtration layer, after which water is injected from the water discharge pipe, thereby preventing clogging of the sand filtration layer. 